The presently disclosed embodiments are directed to the improvement of flexible imaging members used in electrophotography. These embodiments are, more particularly, pertaining to a structurally simplified curl-free flexible electrophotographic imaging member without the need for an anticurl back coating, having a functionally improved top outermost exposed slippery imaging layer which furthers extends service life, and provides a process for making and using the member. More specifically, the present disclosure relates to all types of flexible electrophotographic imaging member belts used in electrophotography.
Electrophotographic imaging members are known in the art. Typical electrophotographic imaging members include (1) electrophotographic imaging members or photoreceptors for electrophotographic imaging systems and (2) electroreceptors such as ionographic imaging members for electrographic imaging systems. Generally, these imaging members comprise at least a supporting substrate and at least one imaging layer comprising a thermoplastic polymeric matrix material. In a photoreceptor, the photoconductive imaging layer may comprise only a single photoconductive layer or multiple of layers such as a combination of a charge generating layer and one or more charge transport layer(s). In an electroreceptor, the imaging layer is a dielectric imaging layer. Electrophotographic imaging members can have a number of distinctively different configurations. For example, they can comprise a flexible member, such as a flexible scroll or a belt containing a flexible substrate. Since typical flexible electrophotographic imaging members exhibit upward imaging member curling after completion of the outermost exposed imaging layer, an anticurl back coating layer is applied to the back side of the flexible substrate support to counteract/balance the curl and provide the desirable imaging member flatness.
Alternatively, the electrophotographic imaging member can also be a rigid member, such as those utilizing a rigid substrate drum. For these drum imaging members, having a thick rigid cylindrical supporting substrate bearing one or more imaging layers, they do not exhibit imaging member curl-up problem, and thus, do not require an anti-curl back coating layer. Consequently, these are not included in the scope of this disclosure.
In the present disclosure, methodology and material reformulations pertaining to structurally simplified flexible electrophotographic imaging member without the need for an anti-curl back coating, having a functionally improved outermost exposed slippery imaging layer which provides an extended useful service-life function, and a process for making and using the member are specified and equality applicable for flexible imaging members in all varieties of form. However, for reason of simplicity, all the disclosed embodiments detailed hereinafter are focused and represented primarily on the electrophotographic imaging members in flexible seamed belt configuration which are for use in electrophotography.
A number of current flexible electrophotographic imaging members are multilayered photoreceptor belts that, in a negative charging system, comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer (CGL), a charge transport layer (CTL), and an optional anti-curl back coating at the opposite side of the substrate support. In such an electrophotographic imaging member design, the CTL is the top outermost layer and is exposed to the environment. The typical flexible electrophotographic imaging members do always exhibit upward curling after completing the application process of a CTL, an anticurl back coating (ACBC) is usually employed on the back side of the flexible substrate support (the side opposite from the electrically active layers) to balance/control the curl and render the imaging member with desired flatness. So, the ACBC is the bottom outermost exposed layer of the imaging member.
The flexible electrophotographic imaging members are typically prepared in a seamed or seamless belt configuration. Flexible electrophotographic imaging member seamed belts are typically fabricated from a sheet which is cut from a web. The sheets are generally rectangular in shape. The edges may be of the same length or one pair of parallel edges may be longer than the other pair of parallel edges. The sheets are formed into a belt by joining overlapping opposite marginal end regions of the sheet. A seam is typically produced in the overlapping marginal end regions at the point of joining. Joining may be effected by any suitable means. Typical joining techniques include welding (including ultrasonic), gluing, taping, pressure heat fusing, and the like. Ultrasonic welding is generally the more desirable method of joining because it is rapid, clean (no solvents) and produces a thin and narrow seam. In addition, ultrasonic welding is more desirable because it causes generation of heat at the contiguous overlapping end marginal regions of the sheet to maximize melting of one or more layers therein to produce a strong fusion bonded seam.
In a typical negative charging machine design, the prepared flexible imaging member seamed belt is mounted over and encircled around a belt support module comprising numbers of belt support rollers and backer bars ready for electrophotographic imaging function. The flexible electrophotographic imaging member seamed belt is imaged by uniformly depositing an electrostatic charge on the imaging surface of the electrophotographic imaging member and then exposing the imaging member to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the imaging member while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking toner particles on the imaging member surface. The resulting visible toner image can then be transferred to a suitable receiving member or substrate such as paper. Therefore, under these normal machine operation conditions in the field, the flexible imaging member seamed belt is in dynamic fatigued cyclic motion during electrophotographic image printing processes. The top outermost exposed CTL of the imaging member belt is therefore constantly in intimate mechanical (such as for example cleaning blade, tab blade, cleaning brush, etc.) and chemical (such as corona effluents from charging devices) interactions with various machine subsystems and components to fatigue and degrade the CTL. These interactions have been seen to degrade and exacerbate the early development of two crucial CTL material failures in the belt, causing copy printout defects to premature cut short its service life prior to reaching the intended belt life target. Moreover, under the machine imaging member belt functioning conditions, the bottom outermost exposed the ACBC is constantly subjected to belt support rollers and backer bars mechanical interactions which thereby promoting on-set of premature ACBC wear and abrasion streaking failures.
Therefore, the cause of material failures associated with the conventional prior art flexible imaging member seamed belts can be identified from two origins and are listed in the following description.
Onset of Charge Transport Layer Failure
Since the top charge transport layer (CTL) of the imaging member belt is the outermost exposed layer, it is contacting and engaged to all electrophotographic imaging subsystems interactions. That means the top exposed CTL surface of the flexible imaging member belt is constantly subjected to physical/mechanical/electrical/chemical species interactions such as the mechanical sliding actions of cleaning blade and cleaning brush, electrical charging devices, corona effluents exposure, developer components, image formation toner particles, hard carrier particles, receiving paper, and the like during dynamic belt cyclic motion. These interactions, particularly the friction force arisen by the sliding action of cleaning blade/brush/tab blade, against the surface of the CTL have been found to cause surface scratching, abrasion, and rapid CTL surface wear; in some instances, the CTL wears away by as much as 10 micrometers after approximately 20,000 dynamic belt imaging cycles. Excessive CTL wear is a serious problem because it causes significant change in the charged field potential and adversely impacts copy printout quality. Another consequence of CTL wear is the decrease of CTL thickness which alters the equilibrium of the balancing forces between the CTL and the ACBC and impacts imaging member belt flatness. The reduction of the CTL by wear causes the imaging member belt to curl downward at both edges. Edge curling in the belt is an important issue because it changes the distance between the belt surface and the charging device(s), causing non-uniform surface charging density which manifests itself as a “smile” print defects on print-out paper copies. Such a print defect is characterized by lower intensity of print-images at the locations over both belt edges. The susceptibility of the CTL surface to scratches (caused by interaction against developer carrier beads and hard particulate from paper debris) has also been identified as a major imaging member functional failure since the scratches manifest themselves as print defects.
To provide desirable photo-electrical activity function, the current CTL (having a high surface energy of about 39 dynes/cm) is formulated with material compositions that are needed to give proper xerographic function. The high surface energy CTL is therefore prone to collect toner residues, dirt/debris particles, and additives from receiving papers. The eventual fusion of these collected species causes the formation of comets and filming over the outer surface of the CTL, further degrading the image quality of printouts. Another problem associated with high CTL surface energy is that it produces high sliding contact friction against the cleaning blade, tab blade, and cleaning brush mechanical action to exacerbate abrasion and wear failures. Moreover, since high CTL surface energy does also impede absolute toner image transfer from imaging member surface to the receiving paper, it is therefore impacting the image quality of printout copies.
Moreover, the need for an ACBC to control the imaging member upward curling and render belt flatness not only does significantly add the imaging member production cost, it has also been found to introduce a set of serious mechanical problems.
Anticurl Back Coating Mechanical Failure
Typical negatively-charged electrophotographic imaging member belts, such as the flexible multiple layered photoreceptor belt designs, are composite made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer (CGL), a charge transport layer (CTL), and a curl control anticurl back coating (ACBC). The CTL is usually the top outermost layer to be coated over the CGL and is applied by solution coating then subsequently followed by drying the wet applied CTL coating at elevated temperatures of about 120° C., and finally cooling down the coated photoreceptor to the ambient room temperature of about 25° C. Therefore, when a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing solution application of the CTL coating and through drying/cooling process, if unrestrained, spontaneous upward curling of the multilayered photoreceptor does occur. This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. Since the CTL in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the CTL does therefore have a larger dimensional shrinkage than that of the flexible substrate support after the eventual photoreceptor web stock cools down to the ambient room temperature. The exhibition of photoreceptor web stock curling up after completion of CTL coating is due to the consequence of the heating/cooling cycles and processing step. Development of the upward curling can be explained by these mechanism: (1) as the web stock carrying the wet applied charge transport layer is dried at elevated temperature, dimensional contraction does occur when the wet CTL coating is losing its solvent during 120° C. elevated temperature drying, but at 120° C. the CTL remains as a viscous flowing liquid after losing its solvent. Since its glass transition temperature (Tg) is at 85° C., the CTL after losing of solvent will flow to re-adjust itself, release internal stress, and maintain its dimension stability; (2) as the CTL now in the viscous liquid state is cooling down further and reaching its glass transition temperature (Tg) at 85° C., the CTL instantaneously solidifies and adheres to the CGL below because it has then transformed itself from being a viscous liquid into a solid layer at its Tg; and (3) eventual cooling down the solid CTL of the photoreceptor web, from 85° C. down to the 25° C. room ambient, will then cause the CTL to contract more than the flexible substrate support since it has about 3.7 times greater thermal coefficient of dimensional contraction than that of the substrate support. This differential in dimensional contraction results in tension strain built-up in the CTL which therefore, at this instant, pulls the photoreceptor web upwardly to exhibit curling. If unrestrained at this point, the photoreceptor web stock will spontaneously curl-up into a 1½-inch roll. To offset the curling, an ACBC is applied to the backside of the flexible substrate support, opposite to the side having the CTL, and render the photoreceptor web stock with desired flatness.
Curling of an electrophotographic imaging member web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a seamed belt. So, to provide desirable flatness, an ACBC, having an equal counter curling effect but in the opposite direction to the applied CTL, is therefore needed at the reverse or back side of substrate support of the imaging member web to provide force balance and control the curl due to the dimensional shrinkage differential caused by mismatch of the thermal contraction coefficient between the substrate and the CTL, resulting in greater CTL dimensional shrinkage/contraction than that of the substrate after the heating/cooling processes of the applied CTL coating. Even though the application of an ACBC is effective to counteract and eliminate the curl, nonetheless the prepared flat imaging member web does have CTL tension stress build-up in it, creating an internal strain of about 0.27% in the layer. The impact of this internal strain build-up in the CTL is very undesirable, because it is additive to the induced bending strain of an imaging member belt as the belt bends and flexes over each belt support roller during dynamic fatigue belt cyclic motion under a normal machine electrophotographic imaging function condition in the field. The summation of the internal strain and the cumulative fatigue bending strain sustained in the CTL has been found to exacerbate the early onset of CTL cracking, preventing the belt to reach its targeted functional imaging life. Moreover, flexible imaging member belt employing an ACBC has added total belt thickness to thereby increase CTL bending strain which then exacerbates the early onset of belt cycling fatigue CTL cracking failure. The cracks formed in the CTL from the consequence of dynamic belt fatiguing are found to manifest themselves into copy print-out defects, which thereby adversely affect the image quality printout on the receiving paper.
In addition to the abovementioned CTL shortfalls in flexible imaging member belt, various belt function deficiencies have also been observed in the common ACBC formulations used in a typical conventional imaging member belt, such as for example, the ACBC does not always providing satisfying dynamic imaging member belt performance result under a normal machine functioning condition; for example, exhibition of ACBC wear and its propensity to cause electrostatic charging-up are the frequently seen problems to prematurely cut short the service life of the imaging member belt, so it requires frequent costly belt replacement in the field. The ACBC wear under the normal imaging member belt machine operational conditions reduces the ACBC thickness, causing the lost of its ability to fully counteract the curl effect as reflected in exhibition of gradual imaging member belt curl up in relationship that depends on the time of belt function in the field. Curling is undesirable during imaging belt function because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, causing non-uniform charging. In addition, developer applicators and the like, during the electrophotographic imaging process, may all adversely affect the quality of the ultimate developed images. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper. Since the ACBC is the bottom outermost exposed backing layer and has high surface contact friction as it slides over the machine subsystems of belt support module, such as rollers, stationary belt guiding components, and backer bars, during dynamic belt cyclic function, these mechanical sliding interactions against the belt support module components not only exacerbate ACBC wear/scratch/streak problems, it does also cause the relatively rapid wearing away of the ACBC to produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Moreover, ACBC abrasion/scratch/streak damage does also produce unbalance forces generation between the charge transport layer and the ACBC to cause micro belt ripples formation during electrophotographic imaging processes, resulting in bands of streak line print defects in output copies to deleteriously impact image printout quality and shorten the imaging member belt functional life.
High contact friction of the ACBC against machine subsystems is further seen to cause the development of electrostatic charge built-up problem. In other machines the electrostatic charge builds up due to contact friction between the ACBC and the backer bars increases the friction and thus requires higher torque to pull the belts. In full color machines with 10 pitches this can be extremely high due to large number of backer bars used. At times, one has to use two drive rollers rather than one which are to be coordinated electronically precisely to keep any possibility of sagging. Static charge built-up in the ACBC increases belt drive torque, in some instances, has also been found to result in absolute belt stalling. In other cases, the electrostatic charge build up can be so high as to cause sparking.
Another problem encountered in the conventional belt photoreceptors using a bisphenol A polycarbonate ACBC that are extensively cycled in precision electrophotographic imaging machines utilizing belt supporting backer bars, is an audible squeaky sound generated due to high contact friction interaction between the ACBC and the backer bars. Further, cumulative deposition of ACBC wear debris onto the backer bars may give rise to undesirable defect print marks formed on copies because each debris deposit become a surface protrusion point on the backer bar and locally forces the imaging member belt upwardly to interferes with the toner image development process. On other occasions, the ACBC wear debris accumulation on the backer bars does gradually increase the dynamic contact friction between these two interacting surfaces of ACBC and backer bar, interfering with the duty cycle of the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases. Additionally, it is also important to point out that an electrophotographic imaging member belts prepared that required ACBC to provide flatness has produced more than the above listed problems, the application of an ACBC does further incur additional material and labor cost impact to the current imaging member production process.
Thus, the conventional prior art flexible electrophotographic imaging member seamed belts comprising a supporting substrate (having a conductive surface on one side, coated over with at least one photoconductive layer (such as having the top outermost exposed CTL) and coated on the other side of the supporting substrate with a conventional ACBC), do have inherent physical/mechanical deficiencies and limits which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers. While the above mentioned electrophotographic imaging member belts may be suitable or partially satisfactory for their intended purposes, further improvement on these imaging member belts are needed. For example, there continues to have the need for improvements in such systems, particularly for an imaging member belt that has: (a) a CTL with reduction of surface contacting friction, superb wear resistance, and lubricity to suppress wear as well as enhancing toner image transfer efficiency, and (b) sufficiently flatness without the need for an ACBC to reduce torque for ease of belt drive, nil or no wear debris generation, as well as elimination of electrostatic charge build-up problem, even for imaging member belt functioning in larger printing apparatuses. With so many of these above mentioned shortcomings and problems associated with the conventional electrophotographic imaging member belts now understood, therefore development of new/improved material reformulation and a methodology for fabricating imaging member belts that produce robust mechanical function and meet the future machine imaging member belt life extension requirement free of issues has been pursued. In the present disclosure, a low surface energy and internal stress/strain relieved CTL material reformulation method and process of making a flexible imaging member belt free of all the above mentioned deficiencies have been successfully identified and demonstrated through the creation of ACBC-free imaging member.
In summary, there is an urgent need for preparation of improved flexible imaging members having robust CTL re-formulation which exhibits little or non of the abovementioned shortfalls and provide good abrasion/wear/filming resistances, surface slipperiness/lubricity, and mechanical durability. Furthermore, the improved imaging member shall also be a curl-free design without the need for an ACBC. In essence, such imaging members redesigned according to the present disclosure shall effect physical/mechanical function enhancements to significantly impact the imaging members' service life extension in the field. Additionally, the flexible imaging members thus prepared to have these improvements are also required to provide effective production cost cutting benefit.
The successful preparation of low surface energy/slippery CTL and curl-free imaging member belt, without an ACBC, does suppress abrasion/wear failure, no tribo-electrical charging, and extend the CTL service life will thereby be described in detailed embodiments hereinafter.
The following patents, the disclosure of which are incorporated in their entireties by reference, are mentioned for background information.
U.S. application Ser. Nos. 12/712,064, 12/434,572, 12/476,200, 12/471,311, 12/551,440, 12/551,414 disclose electrophotographic imaging members having a functionally improved top outermost exposed slippery imaging layer.
U.S. Pat. No. 7,611,811 discloses a negatively charged electrophotographic imaging member comprising a flexible supporting substrate with an electrically conductive outer surface, a CGL, and at least a one outermost exposed CTL layer comprises of a polycarbonate binder, a charge transport compound, and a low surface energy film forming polymer containing short chain polysiloxane segments in its backbone. The prepared imaging member has low surface energy surface, reduced surface contact friction, and improved surface lubricity.
U.S. Pat. No. 6,117,603 discloses an electrophotographic imaging member including a supporting substrate having an electrically conductive outer surface and at least a one layer having an exposed imaging surface, the CTL, including a continuous matrix comprising a film forming polymer and a surface energy lowering liquid polysiloxane.
U.S. Pat. No. 6,326,111 relates to a charge transport material for a photoreceptor including at least a polycarbonate polymer, at least one charge transport material, polytetrafluoroethylene (PTFE) particle aggregates having an average size of less than about 1.5 microns, hydrophobic silica and a fluorine-containing polymeric surfactant dispersed in a solvent. The presence of the hydrophobic silica enables the dispersion to have superior stability by preventing settling of the PTFE particles. A resulting CTL produced from the dispersion exhibits excellent wear resistance against contact with an AC bias charging roll, excellent electrical performance, and delivers superior print quality.
U.S. Pat. No. 6,337,166 discloses a charge transport material for a photoreceptor including at least a polycarbonate polymer binder having a number average molecular weight of not less than 35,000, at least one charge transport material, polytetrafluoroethylene (PTFE) particle aggregates having an average size of less than about 1.5 microns, and a fluorine-containing polymeric surfactant dispersed in a solvent mixture of at least tetrahydrofuran and toluene. The dispersion is able to form a uniform and stable material ideal for use in forming a CTL of a photoreceptor. The resulting CTL exhibits excellent wear resistance against contact with an AC bias charging roll, excellent electrical performance, and delivers superior print quality.
U.S. Pat. No. 4,265,990 illustrates a layered photoreceptor having a separate charge generating layer and a separate CTL. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the CTL. The photogenerating layer utilized in multilayered photoreceptors includes, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Examples of photosensitive members having at least two electrically operative layers including a charge generating layer and a diamine containing transport layer are disclosed in U.S. Pat. Nos. 4,233,384; 4,306,008; 4,299,897; and, 4,439,507, the disclosures of each of these patents being totally incorporated herein by reference in their entirety.
U.S. Pat. No. 5,096,795 discloses the preparation of a multilayered photoreceptor containing particulate materials for the exposed layers in which the particles are homogeneously dispersed therein. The particles reduce the coefficient of surface contact friction, increase wear resistance and durability against tensile cracking, and improve adhesion of the layers without adversely affecting the optical and electrical properties of the resulting photoreceptor.
In U.S. Pat. No. 5,069,993 issued to Robinette et al on Dec. 3, 1991, an exposed layer in an electrophotographic imaging member is provided with increased resistance to stress cracking and reduced coefficient of surface friction, without adverse effects on optical clarity and electrical performance. The layer contains a polymethylsiloxane copolymer and an inactive film forming resin binder.
U.S. Pat. No. 5,830,614 relates to a charge transport having two layers for use in a multilayer photoreceptor. The photoreceptor comprises a support layer, a charge generating layer, and two CTLs. The CTLs consist of a first transport layer comprising a charge transporting polymer (consisting of a polymer segment in direct linkage to a charge transporting segment) and a second transport layer comprising a same charge transporting polymer except that it has a lower weight percent of charge transporting segment than that of the first CTL. In the '614 patent, the hole transport compound is connected to the polymer backbone to create a single giant molecule of hole transporting polymer.
Although the prior arts of all the above electrophotographic imaging members disclosures comprising a flexible supporting substrate, a conductive surface on one side, coated over with at least one photoconductive layer (such as the outermost CTL), and coated on the other side of the supporting substrate with an ACBC have offer some degree of improvements, but do still exhibit deficiencies which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers.
While the above mentioned electrophotographic imaging members may be suitable or limited for their intended purposes, further improvement on these imaging members are definitively required. For example, there continues to be the need for improvements in such systems, particularly for an imaging member belt that does provide reduction in CTL surface contact friction to suppress/minimize wear, imparts lubricity/slipperiness to ease cleanly, minimizes wear debris build-up, and enhances toner image transfer efficiency to receiving papers with improved image copy print-out quality in printing apparatuses and xerographic machines. In the present disclosure, a CTL reformulated by plasticizing the layer and plus the inclusion of a selected low surface energy polymer has been successfully demonstrated. In brief, this is achieved through the inclusion of a novel low surface energy bisphenol A polycarbonate binder to impart CTL surface lubrication with the incorporation of a plasticizing liquid to relieve the CTL internal stress/strain build-up such that the resulting imaging member exhibits little or no upward curling without the need for an ACBC. Therefore, the prepared flexible imaging member has provided surface slipperiness to effect contact friction reduction, abhesiveness for toner image paper transfer efficiency enhancement, and is also curl-free without the application of an ACBC to impact imaging member belt production cost reduction.